Hypoglycemia: This condition is a common cause of lethargy and poor feeding in neonates. Mild acidosis can also be present due to the metabolic effects of low blood glucose levels. Given the clinical presentation, hypoglycemia is a straightforward and likely cause that needs immediate attention.

Congenital Heart Disease: Although the sepsis screen is negative, congenital heart defects can cause acidosis and poor feeding due to decreased cardiac output and subsequent metabolic derangements. Early detection is crucial for the management of these conditions.
Respiratory Issues (e.g., Respiratory Distress Syndrome): Neonates with respiratory distress may exhibit lethargy and poor feeding. Mild acidosis can result from respiratory acidosis if there is hypoventilation.
Neonatal Abstinence Syndrome: If the mother was on certain medications or substances during pregnancy, the neonate could experience withdrawal, leading to irritability, poor feeding, and possibly mild metabolic disturbances.

Inborn Errors of Metabolism: Although less common, conditions like maple syrup urine disease, phenylketonuria, or organic acidemias can present with nonspecific symptoms such as lethargy, poor feeding, and metabolic acidosis. Early diagnosis is critical for the initiation of specific treatments that can significantly improve outcomes.
Congenital Infections (e.g., Toxoplasmosis, Rubella, Cytomegalovirus, Herpes simplex): Even with a negative sepsis screen, congenital infections can have a wide range of presentations, including neurological symptoms and metabolic disturbances. These conditions require prompt diagnosis and treatment to prevent long-term sequelae.
Spinal Muscular Atrophy: This genetic disorder can cause muscle weakness, leading to poor feeding and respiratory issues, which might result in acidosis. Early diagnosis is essential for genetic counseling and supportive care.

Mitochondrial Disorders: These are a group of disorders caused by dysfunctional mitochondria, affecting energy production within cells. They can present with a variety of symptoms, including metabolic disturbances, but are relatively rare and often diagnosed later in life.
Ketotic Hypoglycemia: A condition where the body produces ketones in response to low blood glucose levels, which can cause acidosis. It's less common in neonates but should be considered in the differential diagnosis of hypoglycemia with acidosis.
Endocrine Disorders (e.g., Congenital Adrenal Hyperplasia): Certain endocrine conditions can lead to metabolic disturbances, including acidosis, and require specific diagnostic tests and treatments.